The present invention relates to a circularly polarized cross dipole antenna favorably used as a circularly polarized antenna for communications.
There is Jpn. Pat. Appln. KOKAI Publication No. 04-291806 as a document showing a prior art technique of the circularly polarized cross dipole antenna. This Publication discloses a circularly polarized (cross dipole) antenna for communications which is constituted of a cross dipole antenna element and a reflector.
FIG. 7 is a perspective view schematically showing an example of a prior art circularly polarized cross dipole antenna corresponding to that of the above Publication. The antenna shown in FIG. 7 includes a reflector 110, a pair of L-shaped dipole antenna elements 111 and 112, a parasitic loop 113, and a feeder 114.
The L-shaped dipole antenna elements 111 and 112 are arranged to cross each other and supplied with power through the feeder 114 to radiate a circularly polarized radio wave in the main radiating direction indicated by solid-line arrow M and in its opposite direction indicated by broken-line arrow N. The reflector 110 is disposed at a given distance from the paired L-shaped dipole antenna elements 111 and 112, and reflects the radio wave radiated from the antenna elements 111 and 112 in the opposite direction N and combines it with the radio wave radiated therefrom in the main radiating direction M into a composite wave. The parasitic loop 113 is a metal loop disposed within the same plane as the antenna elements 111 and 112 and has a function of guiding the composite wave in the main radiating direction M.
The foregoing prior art circularly polarized cross dipole antenna has the following problems.
The feeder 114 is constituted of a pair of conductors. One end of each of the conductors is connected to its corresponding bending portion of the antenna elements 111 and 112, and the other ends thereof extend in the direction of the reflector 110, or in the direction perpendicular to the plane including the antenna elements 111 and 112. Since the prior art antenna is constituted three-dimensionally, various problems arise in mounting the antenna on a circuit board.
Since, more specifically, the prior art antenna is difficult to mount on a circuit board compactly because of its three-dimensional structure, it is poor in workability when it is mounted on the circuit board together with a balun (a matching transformer for transforming a balanced line and an unbalanced line) and a matching circuit, and the number of assembling steps is increased. Since, furthermore, the prior art antenna is increased in volume, it is disadvantageous for its transportation and transportation costs.